In a semiconductor device such as an LSI, an impurity diffusion region (e.g., a source/drain region) is formed in a surface layer of a semiconductor substrate, and a thermal oxidation film is formed on the impurity diffusion region as a gate insulating film. At this time, the growth rate of the thermal oxidation film tends to be faster as the impurity concentration of the impurity diffusion region becomes higher. Such a phenomenon is referred to as accelerating oxidation.
The concentration of the impurity diffusion region formed in the semiconductor substrate is optimized depending on a role that the impurity diffusion region plays. Thus, a plurality of impurity diffusion regions in one chip rarely have the same concentration. Normally, concentrations of the impurity diffusion regions are different from one another.
However, when the concentrations of the impurity diffusion regions are different from one another, the thermal oxidation film grows thicker in the impurity diffusion region with a high concentration than in other portions due to the above-described accelerating oxidation. The thermal oxidation film is needed to be removed by etching before a metal silicide layer is formed in the surface layer of the impurity diffusion region. However, when the etching time is adjusted for the thickly formed thermal oxidation film, the device isolation insulating films located below the thermal oxidation film are also etched in the portions where the thermal oxidation film is formed thinly.
For this reason, as shown in Japanese Patent Application Laid-open Publication No. 2003-282740 for example, a problem arises that a leak current increases in edge portions of an active region of a transistor.
In Japanese Patent Application Laid-open Publication No. 2003-282740, for the purpose of avoiding such a problem, a thermal oxidation film is covered by a silicon nitride film in order to prevent the thermal oxidation film under the silicon nitride film from being acceleratedly oxidized (see, paragraph 0040).
In addition, in Japanese Patent Application Laid-open Publication No. 2002-280464, a substance such as nitrogen having a function to prevent accelerating oxidation is ion-implanted into a semiconductor substrate (see paragraph 0060).